Process and solution for bleaching photographic images



United States Malt Patented Oct. 30, 1962 3,061,433 PROCESS AND SOLUTION FOR BLEACHING PHOTOGRAPHIC IMAGES Robert Charles Morris Smith, llford, England, and

Eustace Raymond Towuley, Hillington, Glasgow, Scotland, assignors to llford Limited, Ilford, England, a British company No Drawing. Filed Aug. 14, .1959, Ser. No. 833,671 9 Claims. (Cl. 96-60) The present invention concerns chemical solutions for use in the development and processing of exposed photographic film in continuously operating rapid processing photographic projectors in which continuous 24-hour operation is a factor of considerable importance.

This application is a continuation-in-part of application Serial No. 752,928, filed August 4, 1958.

It is an object of the invention to provide means in photographic developer and bleach solutions to reduce or eliminate the formation of precipitants in rapid photographic processing which build up and block fluid fiow through jet sprays having small orifices.

Rapid processing photographic projectors of the kind here concerned can develop and project a photograph in as short a time as two seconds. Where better resolution and archival quality are required, black and white processing requires from four to six seconds and colour processing presently requires about fifteen seconds.

The physical structures of rapid processing photographic projectors intended for continuous operation with which the present invention is intended to be used are disclosed in co-pending application Serial No. 642,698, titled Processing of Photographic Material, filed February 27, 1957. Reference is also made to a brochure published by Kelvin & Hughes Limited, New North Road, Barkingside, Essex, England, entitled: Rapid Processing Photographic Projector.

In the aforesaid apparatus the exposed photographic material is subjected to successive processing solutions whereby a positive image is formed in the material and the material is immediately passed through an optical projection section whereby the projected positive image may be viewed. To arrive at a positive image it is necessary that the processing should be by the so-called reversal method, is. the initial latent image resultant on light exposure is developed to a negative record in silver; this silver is bleached out and the residual silver halide is developed. Moreover, where a colour image is to be obtained it is necessary to bleach out the developed silver (existing either as negative or positive image).

The processing solutions are applied by means of jet applications operating in a single processing zone. This leads to some problems. The developers used are normally highly alkaline and the most eifective silver bleach baths are those based on chromic acid. Hence, after the first jet has sprayed developer the walls of the processing zone and the surfaces of the jet unit become contaminated with highly alkaline developer which, when followed by the use of the chromic acid bleach solution, slowly results in a buildup of a complex chromium hydroxide sludge which has been found to cause severe blockage of the jets themselves and interfere with the quality of the processing due to its formation on the interior surfaces of the processing zone. In some cases the machine ceases to function.

This problem is not normally encountered with conventional dish processing since the photographic emulsion is usua'lly developed, thoroughly washed and subsequently placed in a separate bath of bleach solution and thus developer and the acid bleach.

Normally, it is necessary to utilise an oxidising acid such as the chromic acid previously mentioned in order successfully to dissolve the silver image in a short period of time. This oxidising acid usually consists of a mixture of sulphuric acid and potassium dichromate in various proportions which gives a complex oxidising acid containing chromic acids, dichromic acids, etc. The precise mechanism of this chemical reaction is not agreed to by all the various authorities.

It will be readily seen that in developer solution containing a large amount of hydroxyl ions, caustic soda will react with the chromium salts and form various complex chromium hydroxides, and it is these compounds which cause trouble, slowly but inevitably, as a result of the successive application of each processing cycle, although only a minute amount of precipitation is caused on each occasion. In practice, it is found that the equipment will work satisfactorily for a few days and after this the successful operation is very much interfered with or interrupted by the deposited sludge described above.

In order to solve this problem we investigated the working of the solution and found that if an addition of acetic acid is made to the solution, this sludge formation can be so much minimised as to make the operation of the equipment in the same way hitherto described completely successful. A small mount of sludge is still formed under these conditions, but the amount is negligible and appears not to matter.

According to the present invention, therefore, there is provided a bleach solution for use in rapid processing photographic projectors in which the elapsed time from exposure to projection is less than thirty seconds, consisting of potassium dichromate, acetic acid, and sulphuric acid so proportioned that photographic film is rapidly processed with the production of a negligible amount of sludge and the elimination of staining in the event of over-bleaching.

Preferably the bleach solution characteristic of this invention contains, in aqueous solution, 0.5 to 5% by weight potassium dichromate, 0.5 to 10% by weight acetic acid and 2 to 15% by weight sulphuric acid. Where the processing time is 2 to 6 seconds it is preferred that the solution should contain less than 2% potassium dichromate, lessthan 4% acetic acid, and less than 6% sulphuric acid.

In particular there may be used an aqueous solution comprising:

Percent by weight Various changes in the constitution of each of these substances are permissible over the range indicated and still give satisfactory results. However, the particular solution just set forth is probably optimum for vuse in the above'equipment as it also provides the correct bleaching time and avoids most of the staining difficulties associated with such solution when the film emulsion is over-bleached.

It is believed that the addition of acetic acid to the chromic acid solution as outlined above makes all the difference between success and failure in the continuous operation of rapid processing photographic projectors of the type under consideration.

The following example will serve to illustrate the invention:

EXAMPLE Photographic colour film comprising a silver halide emulsion containing a coupling compound which, on de- Jet 1.Develper Hexamethylene phosphate (the commercial product Calgon (in 20% solution) mls 15 Trisodium phosphate (cryst.) g 80 Sodium sulphite anhydrous in aqueous solution) mls 50 Potassium bromide (10% aqueous solution) rnls l0 Diethylaminotoluidine g 4 1-phenyl-3-pyrazolidone (0.625 g. +6 ml. of 10% anhydrous sodium sulphite in water, made up to 100 cc. with water) mls 10 Water to make, 1 litre.

Jet 2.Bleaching Solution Potassium dichromate (cryst.) g 10 Sulphuric acid (conc.) g 20 Glacial acetic acid g 50 Water to make, 1 litre.

Jet 3.Fixing Solution Ammonium thiocyanate (cryst.) Dye (Soluble Blue N) Water to make, 1 litre.

Jet 4.-Wash Solution 2% aqueous sodium thiosulphate (cryst.)

The aforesaid type of material yields images in colour on a white background.

While there have been described above what are presently believed to be the preferred forms of the invention, variations thereof will be obvious to those skilled in the art and all such changes and variations which fall within the spirit of the invention are intended to be covered by the generic terms in the appended claims, which are variably worded to that end.

What we claim is:

1. A bleach solution for use in rapid processing photographic projectors in which the elapsed time from exposure to projection is less than thirty seconds, consisting of potassium dichromate, acetic acid, sulphuric acid and water so proportioned that photographic film is rapidly processed with the production of a negligible amount of sludge and the elimination of staining in the event of over-bleaching, the percentage of potassium dichromate being at least half a percent and not more than five percent by weight, of anhydrous acetic acid being at least half a percent and not more than ten percent by weight, of anhydrous sulphuric acid being at least two percent and not more than fifteen percent by weight, and the balance being water to make up one hundred percent.

2. A bleach solution for use in rapid processing photographic projectors in which the elapsed time from exposure to projection is less than thirty seconds, consisting of potassium dichromate, acetic acid, sulphuric acid and water so proportioned that photographic film is rapidly processed with the production of a negligible amount of sludge and the elimination of staining in the event of over-bleaching, the percentage of potassium dichromate being not more than two percent by weight, of anhydrous acetic acid being not more than four percent by weight, of anhydrous sulphuric acid being not more than six percent by weight and of water being at least eight-eight percent by weight whereby the processing time of said bleach solution is between two and six seconds.

3. In a process for the rapid processing of photographic materials enroute from an exposure position to a projection position with a time lapse of less than thirty seconds from exposure to projection, the step of bleaching a developed silver image by treating the photographic material with a bleach solution consisting of potassium dichromate, acetic acid, sulphuric acid and water in which the percentage of potassium dichromate is at least half a percent and not more than five percent by weight, anhydrous acetic acid is at least half a percent and not more than ten percent by weight, anhydrous sulphuric acid is at least two percent and not more than fifteen percent by weight, and the balance is water to make up one hundred percent.

4. The process set forth in claim 3, in which the percentage of potassium dichromate is not more than two percent by weight, of anhydrous acetic acid is not more than four percent by weight, of anhydrous sulphuric acid is not more than six percent by weight and of water at least eighty-eight percent by weight whereby the processing time of said bleach solution is between two and six seconds.

5. The process set forth in claim 4, in which the percentage of potassium dichromate is substantially one percent by weight, of anhydrous acetic acid substantially two percent by weight, of anhydrous sulphuric acid substantia'lly three percent by weight, and water substantially ninety-four percent by weight whereby the elapsed time from exposure to projection is less than thirty seconds.

6. In a process for the rapid processing of photographic materials the steps of exposing the photographic material, developing a silver image by treating the photographic material with a developer solution, bleaching the developed silver image by treating the photographic material with a bleach solution consisting of potassium dichromate, acetic acid, sulphuric acid and water in which the percentage of potassium dichromate is at least half a percent and not more than five percent by weight, anhydrous acetic acid is at least half a percent and not more than ten percent by weight, anhydrous sulphuric acid is at least two percent and not more than fifteen percent by weight, and the balance is water to make up one hundred percent, washing the photographic materials with a wash solution and drying the photographic materials, all with a time lapse of less than twenty seconds.

7. The process set forth in claim 6, in which the percentage of potassium dichromate is not more than two percent by weight, of anhydrous acetic acid is not more than four percent by weight, of anhydrous sulphuric acid is not more than six percent by weight and of water at least eighty-eight percent by weight whereby the processing time of said bleach solution is between two and six seconds.

8. The process set forth in claim 7, in which the percentage of potassium is substantially one percent by weight, of anhydrous acetic acid substantially two percent by weight, of anhydrous sulphuric acid substantially three percent by weight and water substantially ninetyfour percent by weight whereby the elapsed time from exposure to projection is less than thirty seconds.

9. The process set forth in claim 8, and the step of projecting the dried photographic material all with a time lapse of less than twenty seconds.

References Cited in the file of this patent UNITED STATES PATENTS 1,930,140 Becker Oct. 10, 1933 2,195,419 McQueen et al. Apr. 2, 1940 2,326,055 Morris Aug. 3, 1943 2,507,183 Wilder et al. May 9, 1950 OTHER REFERENCES American Photographic Publishing (30., Boston (1947). 

3. IN A PROCESS FOR THE RAPID PROCESSING OF PHOTOGRAPHIC MATERIALS ENROUTE FROM AN EXPOSURE POSITION TO A PROJECTION POSITION WITH A TIME LAPSE OF LESS THAN THIRTY SECONDS FROM EXPOSURE TO PROJECTION, THE STEP OF BLEACHING A DEVELOPED SILVER IMAGE BY TREATING THE PHOTOGRAPHIC MATERIAL WITH A BLEACH SOLUTION CONSISTING OF POTASSIUM DICHROMATE, ACETIC ACID, SULPHURIC ACID AND WATER IN WHICH THE PERCENTAGE OF POTASSIUM DICHROMATE IS AT LEAST HALF A PERCENT AND NOT MORE THAN FIVE PERCENT BY WEIGHT, ANHYDROUS ACETIC ACID IS AT LEAST HALF A PERCENT AND NOT MORE THAN TEN PERCENT BY WEIGHT, ANHYDROUS SULPHURIC ACID IS AT LEAST TWO PERCENT AND NOT MORE THAN FIFTEEN PERCENT BY WEIGHT, AND THE BALANCE IS WATER TO MAKE UP ONE HUNDRED PERCENT. 